1. Field of the invention
The present invention relates to a radius of curvature detecting apparatus and method for calculating a radius of curvature of a curve of a road based on road information obtained from a navigation system and the like.
2. Prior Arts
In recent years, various kinds of electronically controlled safe driving apparatuses employing a vehicle navigation system and the like have been developed. In those apparatuses, there are safe driving techniques in which a radius of curvature of a curve ahead is calculated based upon information on maps obtained from the vehicle navigation system, and based on the calculated radius and a vehicle speed before entering into the curve, an alarm is issued to warn a vehicle driver to reduce the vehicle speed or a brake is applied automatically so as to reduce the vehicle speed up to a proper safe speed, if it is judged that the vehicle will not be able to negotiate the curve.
Whether those apparatuses are practicable or not in motor vehicles depends on how the calculated or estimated radius of curvature is proximate to that of an actual curve. For example, as shown in FIG. 6a, a point T referred to as a curve detecting point is considered on a vehicle pass line RL ahead of the vehicle. Then, a first point S is placed at a point going back from the point T by a sampling distance Lj which is established based on a radius of curvature of a previous curve. Next, a second point U is placed at a point going forward from the point T by an equal distance Lj. After that, a first vector ST drawn from the first point S to the curve detecting point T and a second vector TU drawn from the point S to the point U form an angle .theta. which is referred to as a "curvature index" at the curve detecting point T. Thus, a radius of curvature R of the curve is calculated according to the following equation: EQU R=Lj/(2.multidot.sin (.theta./2))
Such calculation method is disclosed in Japanese Patent Application Laid-open No. Toku-Kai-Hei 8-194886.
The road data contained in maps obtained from the vehicle navigation system are constituted of points data inputted at discretionary intervals and lines data formed by connecting two adjacent points successively, as illustrated in FIG. 5.
According to this prior art, it arouses a problem of accuracy to apply inherent points data of the navigation system to the calculation of radii of curvature and therefore, as shown in FIG. 6b, interpolated points must be placed between the curve detecting point T and known data from the navigation system.
Further, as shown in FIG. 6c, in a case where the road bents complicatedly, depending on the way of establishment of the sampling distance Lj, it is impossible to link respective curve detecting points smoothly, so that the obtained road configuration presents a great difference from an actual one and as a result an accurate radius of curvature of the curve is unable to be produced.
Further, in order to estimate the road configuration, since the vehicle must pass through the three points S, T and U described above, a rather large radius of curvature tends to be calculated, this permitting the vehicle to enter into the curve at a vehicle speed larger than the limited vehicle speed without issuing an alarm or operating safe driving apparatuses.